What Students Actually Understand About Compound Interest

Here is something we are certain of after designing this curriculum: a student can understand compound interest better than most adults, as long as you sequence it correctly.

Start with the doubling game, not the formula

We never open with the formula. We open with a choice: would you rather have a million dollars today, or a penny that doubles every day for thirty days? The class will vote. Almost everyone will take the million. Then we do the math together on the board, day by day, and somewhere around day twenty eight, we expect the room to go quiet. That penny is worth over five million dollars. Nobody will have taught them a formula. They will have discovered exponential growth, where small, steady gains accelerate into explosive ones, all by themselves. They own it.

The formula comes later. By then it will be just notation for something they already feel.

The hard part is not the math, it is the timeline

The honest hard part of compound interest for a student is not the arithmetic. It is believing in a future self. Students this age live in a two week time horizon. So we make the timeline concrete: we show them what happens to money over the next several years, a future they can actually picture, and then over the decade after that, the stage of life the people they already admire are in.

The mistake most curricula make

Schools tend to introduce compound interest in high school, just before students can legally open accounts. That is still too late for the idea to feel like a discovery, and just in time for it to feel like a test topic. Younger students are at the perfect age: the math is within reach, and the intuition sticks for life. When a student who played the doubling game hears about a savings account years later, they will not remember a definition. They will remember the room going quiet on day twenty eight.